It is well known that the voltage output from a mains supply can vary significantly, in particular in circumstances where the power demanded from the mains supply is greater than that which the supply can deliver.
For applications which require a constant power input, the reduction in mains voltage may be countered by an increased current drawn from the mains. Such may be the case for switched mode power supplies (SMPS), which are typically controlled in order to maintain a constant output power. The situation of mains voltage being considerably lower than the nominal value is known as “brown-out”.
Since the increased current can lead to overheating of components or electrical overstress, it is known, in conventional systems, to switch off the application, in this case the switched mode power supply, when the mains voltage drops below a certain predetermined or preset value. It is also known to provide hysteresis in such brown-out protection arrangements, in order to prevent the current oscillations where for example the SMPS repeatedly is switched on and off each successive mains half cycle.
Such brown-out protection provides effective protection for the SMPS against under-voltage conditions; however, switching the SMPS off in all under-voltage situations as soon as the voltage falls below the preset value, irrespective of whether an over-current would actually have occurred, may result in an “overcautious” method of protection, with unnecessarily disturbance to the user.